This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In a TDMA communication system, a base unit facilitates communication between other base units and multiple local mobile terminals (MTs), which may also be referred to as handsets. The base unit and the mobile terminals are typically capable of transmitting and receiving a data signal at a particular frequency or group of frequencies. The data signal is broken into a number of smaller increments known as time slots, which may recur during each cycle of the data signal. Using TDMA, multiple data transmission sessions can take place simultaneously. During a given communication session, a mobile terminal may be assigned a particular time slot. The data from that mobile terminal may be transmitted in the assigned time slot for the duration of a communication session. For a given base station environment, it is typical for at least some TDMA timeslots to be unused at a given time.
TDMA may be used in spread spectrum communication systems, such as DSSS systems. In a DSSS system, the original data signal is spread by multiplying it with a wideband spreading code. Spreading converts a narrowband signal with a relatively high power spectral density into a wideband signal that has a low power spectral density. That is, the energy of the signal is spread out over a wide frequency range. A DSSS signal is often below the noise floor.
A DSSS receiver is able to process the signal because of the correlation gain from correlating against the spreading code at the receiver. Because of their low power spectral density, DSSS signals are often hard to detect and cause very little interference with other signals in that frequency band.
A typical method of synchronizing a TDMA structure that is being received is to decode the payload data contained in data packets associated with the various time slots to determine the reference point in the TDMA structure of the decoded packet. This scheme generally requires a receiving system to, first, reliably detect the packet boundaries and then to be able to demodulate, decode and otherwise process the packet to extract the relevant TDMA information. Such initial packet boundary detection may be difficult to perform, especially in conditions where signal-to-noise ration (SNR) SNR is low.
An example of the typical method is to correlate the received data signals with an appropriate pseudo-noise (PN) sequence to determine the correlation peak locations. When the correlation peak locations are known, packet boundaries may be locked onto and payload data decoded based on the location of the data relative to the identified packet boundary. In such a scheme, the detection of the correlation peaks does not provide any information about the TDMA structure until the payload data is decoded using forward error correction (FEC) technology. An apparatus and method that allows a reliable determination of the TDMA structure of a data signal in the correlation domain without decoding the FEC-encoded payload data, thus speeding up the TDMA acquisition and improving its accuracy, is desirable.